This past week I've been working on my Fuel gauge. The problem is that I turn the key to switched power (run position) and the Fuel gauge will read accurate but after some time (sometimes 30 seconds and sometimes 15 minutes) the needle will slowly start to wander. For instance, I have 1/2 tank of fuel and it will wander to 3/4 tank and then back a little until going to the full position and sitting there for a while before wandering back to 1/2 tank.

Little background: I have a full set of Speedhut aftermarket gauges. They all worked fine when installed. I had an interior fire that caused me to replace nearly the entire interior. The gauges only had smoke damage and only needed the clear lens replaced. None of their wiring was harmed. I did however have to replace the entire interior wiring harness. All electronics work flawlessly now, except this single problem.

Right now I have 1/2 tank fuel in the tank and I know this because I filled the tank with that many gallons. I also measured the resistance between the sending unit and ground at the tank's connector and I get about 48 ohms. The factory fuel sending unit is 0-90 ohms resistance so it is indeed 1/2 tank. I have also measured the sending unit resistance at the connection to the gauge cluster and at the back of the gauge and it is the same. For each location that I check the sending unit wire I am also checking it against the ground wire it is referencing. At every spot it measures 1/2 tank. I've also made a spare ground wire from a body ground and used it to further ground out the ground wire the gauge is referencing to find out if that changes the reading, which it doesn't.

I have removed and recleaned the grounds that the electronics are using. There is no paint, it is a clean metal to metal connection. The larger grounds use a star washer while the smaller grounds that have the 8mm head do not have star washers but are a clean and tight metal to metal connection. If I remove the ground the gauge goes to the open position of way past full and if I ground out the signal wire it goes to empty.

The Fuel sending unit is a newer unit, only a few years old. Battery connections are tight and body ground connections are tight.

I've had a few conversations with Speedhut about this and they tell me, repeatedly, that it is a ground issue and I need to (more or less) redo the grounds or ground the gauge to the spot where the fuel pump grounds. I am starting to disagree. Any advice on what to try next?

 

Coupla things to try:

You could run a wire from the block or from one of the points around the motor where a ground strap goes to the chassis, back to the ground point for the pump. I.e. supplement the chassis' conductivity with a wire. I'd suggest a nice large one, like #10. Who knows, might even make your tail/brake lights work better, given that they ground to the same sheet of metal. (or very close to it at least) That would completely eliminate grounding as a potential culprit.

You can get yourself a 100 ohm pot such as https://www.digikey.com/product-deta...2154-ND/203775, disconnect the sending unit, and alligator-clip-lead it to the wiring at both the tank end and the gauge end, and verify that the gauge is working right; by doing so, you split the system up into its parts and determine which part the fault is in; either the sending unit, or the gauge, or the wiring.

 

Finally got around to your first suggestion. I ran a 10 gauge wire from the back of the cylinder head to a body ground on the interior of the car at the front of the transmission tunnel and from there it ran to the actual metal frame of the dash as well as the spot where the fuel pump grounds to the transmission tunnel below the center console. It didn't seem to make a difference. I turned the key on and the gauge sat at empty for a little bit (10-15 seconds) before quickly sweeping over to the correct reading. Tomorrow I'll leave the key on for a while while I am in the garage to observe. It could be that the fuel gauge has to form an average reading and the window to average is 10 seconds. If this doesn't work I will run a new dedicated ground (as opposed to the factory's shared ground) to the gauges. Although with the dash frame now having an added ground I don't see this helping unless it's a problem with a connection.

 

Well I was able to leave the ignition on and observe the fuel gauge while I was tinting the windows. The added supplemental ground strap did not fix the issue so I added a wire from the new 10 gauge ground to the gauge's ground wire. Still didn't fix the problem but I was at least able to totally rule out grounding as a source of the problem.

My next idea was to back probe the connector between the body and the fuel tank and wait for it to begin to stray. When it began to stray I could apply a full ground to the signal wire and the gauge should read empty. So i put in the back probe(s) on the wire(s) and waited. And waited, and waited. Nothing happened. This is leading me to believe that the issue may be a slightly loose pin in that body connector. I'll watch it again this week when I change the ball joints and if it doesn't move I'll know to replace the connector.